THE YOUNG SCIENTIST. 
19 
always found in bottle glass and common 
window glass, render them useless for 
optical purposes. We must, therefore, 
have recourse to plate glass, which is a 
kind of crown glass, and flint glass. Pieces 
of plate glass may be found everywhere, 
and often of excellent quality, and with 
very well polished surfaces. Pieces of 
old plate glass are generally so scratched 
that the surfaces are useless until re- 
ground and polished, but if a good piece 
of new glass plate can be had, very excel- 
lent plano-convex magnifiers may be 
made out of it by grinding and polishing 
the convex side of the lens. For all 
ordinary lenses good plate glass is the 
best material, excepting, of course, the 
regular optical glass. It should be chosen 
of as light a color as possible, and any 
pieces in which streaks can be detected 
should be at once rejected. Sir David 
Brewster recommends, for non-achrom- 
atic lenses, glass of a straw color, whose 
dispersive power is as small as possible. 
The color of a piece of glass is of course 
best seen by looking through it edgewise. 
Flint glass of fair quality may be found 
in many articles of every-day use. Tum- 
blers and goblets of the better class are 
made of flint glass, and we know a very 
fair achromatic telescope, the objective of 
which was partly made out of a large flint 
glass tumbler. Most articles of cut glass 
flint glass would have the shortest focus 
and would magnify most. 
Very excellent pieces of flint glass, but 
of small size, are sometimes found in the 
pendants of chandeliers. 
TOOLS REQUIRED. 
The tools required for mere glass grind- 
ing are few and simple. Large lenses, 
such as are used for telescopes, may be 
ground by hand, but small ones are always 
ground in turning lathes. For this pur- 
pose a very simple and cheap tool will 
answer— even those cheap lathes which are 
sold for from five to ten dollars. We have 
known very good lenses ground and pol- 
ished by means of a lathe which had been 
worked up out of some old materials and 
fitted together with chisels and files. But 
for setting the lenses so as to produce an 
objective that will be better than a mere 
triplet, a really good lathe is necessary. 
A tolerable lathe is also necessary for 
turning up the brass tools used for grind- 
ing and polishing the glass, and as these 
tools must be very true, time is saved and 
a better result obtained by the use of a 
good lathe. Those who possess a well- 
made lathe will find no difficulty in 
making all the tools that are necessary- 
even those small lathes which are best 
adapted to the making of very minute 
lenses. Mr, Wenham describes that used 
by him as follows : 
1.— LATHE FOR GEINDING SMALL LENSES. 
are made out of flint, because this ma- 
terial is more easily worked, and it is also 
more brilliant, owing to its greater re- 
fracting power. Of two lenses of precisely 
the same curvature, one made of flint 
glass and the other out of crown glass, the 
" For the longest radii and lowest powers 
the ordinary foot-lathe is suitable, but 
this is not so well adapted for grinding 
and polishing very minute lenses. A bow 
lathe, such as is used by watchmakers for 
heading their screws and other purposes, 
